Print head assembly, screen printing system and method

ABSTRACT

A print head assembly ( 119 ) for printing a print medium (PM) onto a workpiece (W) through a printing screen ( 105 ) which includes a screen element having a pattern of printing apertures therein, the assembly comprising: first and second carriages ( 141 ) which are in use movably supported on respective ones of first and second guides ( 117 ); and first and second print head units ( 143, 145 ) which are mounted in adjacent, parallel relation to and between the carriages, wherein the print head units each comprise first and second print head actuators which are mounted on respective ones of the carriages, and a support member ( 151 ) which extends between the print head actuators, such as to be movable vertically thereby, and in use supports a printing element.

The present invention relates to a print head assembly for printing aprint medium, such as a fluid ink or a solder paste, onto workpieces,including flexible or delicate substrates, for example, as employed insolar or fuel cell applications, through a printing screen whichincludes a pattern of printing apertures corresponding to a pattern ofdeposits to be printed on a workpiece, a screen printing systemincorporating the same, and a screen printing method.

Very many screen printing systems and print heads have been developedfor the printing of print media onto workpieces.

FIGS. 1 and 2 illustrate a typical screen printing system which utilizesa print head in the form of a squeegee.

The screen printing system comprises a rigid support frame 3, whichsupports a printing screen 5, a pair of spaced, parallel linear bearings7, 7 which are mounted to the support frame 3 at opposite sides of theprinting screen 5, a print carriage 9 which includes a gantry 11 whichis supported on the linear bearings 7, 7, and a print head assembly 15which is mounted to the gantry 11 and operable to print a print mediumthrough a pattern of printing apertures in the printing screen 5 onto aworkpiece which is supported therebelow, and a drive unit 17, whichcomprises a drive motor 19 and a toothed drive belt 21 which is coupledto the print carriage 9, for reciprocatingly driving the print carriage9 along the linear bearings 7, 7.

The print head assembly 15 comprises first and second squeegees 23, 25,which are juxtaposed in parallel relation and independently verticallymovable in order to allow for configuration in dependence upon thedirection of printing, and a drive mechanism 27 which couples thesqueegees 23, 25 to the gantry 11 by a spring coupling (not illustrated)and is operable vertically to raise and lower the squeegees 23, 25 andmaintain a downward force on the operative one of the squeegees 23, 25.The drive mechanism 27 comprises first and second actuators 31, 33, herestepper motors coupled by leadscrews, which are operable vertically toraise and lower respective ones of the squeegees 23, 25.

Operation of the print head assembly 15 is illustrated in FIGS. 2( a)and (b). As illustrated in FIG. 2( a), where printing is in a firstdirection D1 by the first squeegee 23, the first actuator 31 is operatedto drive the first squeegee 23 downwardly into contact with the printingscreen 5 and raise the drive mechanism 27 against the bias of the springcoupling, such that a predetermined downward biasing force is applied tothe first squeegee 23, and the second actuator 33 is operated to drivethe second squeegee 25 upwardly, such that the lower edge of the secondsqueegee 25 is raised clear of the face of the first squeegee 23 inorder to allow for a clear space forwardly of the first squeegee 23 inthe first direction of printing D1, which is necessary to allow forrolling of the print medium ahead of the first squeegee 23. Similarly,as illustrated in FIG. 2( b), where printing is in the other, seconddirection D2 by the second squeegee 25, the second actuator 33 isoperated to drive the second squeegee 25 downwardly into contact withthe printing screen 5 and raise the drive mechanism 27 against the biasof the spring coupling, such that a predetermined downward biasing forceis applied to the second squeegee 25, and the first actuator 31 isoperated to drive the first squeegee 23 upwardly, such that the loweredge of the first squeegee 23 is raised clear of the face of the secondsqueegee 25 in order to allow for a clear space forwardly of the secondsqueegee 25 in the second direction of printing D2, which is necessaryto allow for rolling of the print medium ahead of the second squeegee25.

This screen printing system has operated very successfully, but thepresent inventor has recognized that this screen printing system suffersfrom a number of drawbacks, particularly in relation to the printing offluid inks onto flexible and delicate workpieces. In particular, thebulk of the gantry 11, which is necessary to confer the requiredrigidity, and the weight of the drive mechanism 27 which is mounted togantry 11, limits the speed of movement of the print head assembly 15and hence the machine throughput, and also induces vibrations at thestart and end of each print stroke, which vibrations are particularlydetrimental in relation to the printing of fluid inks onto flexible anddelicate workpieces. In addition, the bulk of the gantry 11 restrictsthe view of the printing screen 5.

Enclosed print heads have also been developed. Examples include theapplicant's earlier WO-A-1998/016387 and WO-A-2001/005592, where thesescreen printing heads are sold under the ProFlow® brand.

These enclosed print heads have operated very successfully to date,particularly in the printing of solder pastes onto printed circuitboards, but new applications, particularly in the production of solarand fuel cells, which utilize flexible and delicate substrates andrequire the printing of fluid inks, are presenting needs which cannot bemet by these existing screen printing heads.

It is thus an aim of the present invention to provide an alternativeprint head assembly for printing a print medium onto workpieces, whichat least in one embodiment allows for the printing of flexible anddelicate workpieces and also allows for the printing of fluid printmedia, such as liquid inks.

In one aspect the present invention provides a print head assembly forprinting a print medium onto a workpiece through a printing screen whichincludes a screen element having a pattern of printing aperturestherein, the assembly comprising: first and second carriages which arein use movably supported on respective ones of first and second guides;and first and second print head units which are mounted in adjacent,parallel relation to and between the carriages, wherein the print headunits each comprise first and second print head actuators which aremounted on respective ones of the carriages, and a support member whichextends between the print head actuators, such as to be movablevertically thereby, and in use supports a printing element.

In one embodiment the guides comprise horizontal guides, preferablylinear bearings.

In one embodiment the print head actuators of each print head unit areoperable to provide for a controlled downward force to the respectivesupport member, thereby enabling application of a controlled downwardforce to a printing element when attached to the support member.

In one embodiment the controlled downward force is in the range of from0 to about 15 N, preferably with an accuracy of less than about 1%.

In one embodiment the print head actuators comprise low-frictionactuators.

In one embodiment the print head actuators comprise low-frictionpneumatic actuators.

In one embodiment the print head actuators each comprise a cylinder, afloating piston which is movably disposed within the cylinder andincludes an attachment element which is attached to one end of thesupport member, and at least one fluid port to which a fluid pressure isapplied to one of raise or lower the floating piston.

In one embodiment the print head units each further comprise verticalguides, preferably linear bearings, which guide the respective supportmember vertically, such that substantially no horizontal force componentis transmitted to the respective print head actuators.

In one embodiment the support member comprises a substantially U-shapedmember.

In one embodiment the U-shaped support member defines a substantiallyclear opening when viewed laterally.

In one embodiment the support member includes first and second bracketelements which are coupled to respective ones of the respective firstand second print head actuators and extend vertically adjacentrespective inner sides of the print head actuators, and a horizontal barelement which extends between the opposite, lower ends of the bracketelements.

In one embodiment the print head units each comprise a stop adjustmentmechanism, which provides for setting of upper and lower limits tovertical movement of the respective print head actuators.

In one embodiment the stop adjustment mechanism comprises first andsecond stop adjusters, preferably screw adjusters, at each end of thesupport member, with the first stop adjusters setting a lower limit tovertical movement of the respective print head actuators and the secondstop adjusters setting an upper limit to vertical movement of therespective print head actuators.

In one embodiment the second stop adjusters can be dis-engaged to allowthe respective support member to be raised well clear of the printingscreen, thereby facilitating removal of the printing screen.

In one embodiment the support members of the print head units aredisposed in spaced relation, such as to allow for introduction of printmedium from above therebetween.

In one embodiment the support member of the first print head unit has asqueegee blade attached thereto and the support member of the secondprint head unit has a flood bar attached thereto, and the assembly ismovable in oppositely-directed print and return strokes, such that, inthe print stroke, the print head actuators of the first print head unitare controlled to bring the squeegee blade into operative contact withthe screen element and apply a required downward force thereto forprinting and the print head actuators of the second print head unit arecontrolled to raise the flood bar into an inoperative position, and, inthe return stroke, the print head actuators of the first print head unitare controlled to raise the squeegee blade to an inoperative positionand the print head actuators of the second print head unit arecontrolled to lower the flood bar to an operative position just clear ofthe screen element, such as to provide for application of a film of theprint medium over the surface of the screen element.

In another embodiment the support members of the print head units eachhave a squeegee blade attached thereto, and the assembly is movable infirst and second oppositely-directed print strokes, such that, in thefirst print stroke, the print head actuators of the first print headunit are controlled to bring the squeegee blade attached thereto intooperative contact with the screen element and apply a required downwardforce thereto for printing and the print head actuators of the secondprint head unit are controlled to raise the squeegee blade attachedthereto just clear of the screen element, and, in the second printstroke, the print head actuators of the first print head unit arecontrolled to raise the squeegee blade attached thereto to a positionjust clear of the screen element and the print head actuators of thesecond print head unit are controlled to lower the squeegee bladeattached thereto into operative contact with the screen element andapply a required downward force thereto for printing.

In another aspect the present invention provides a screen printingsystem, comprising: a support frame which supports a printing screen andincludes first and second guides; the above-described assembly which issupported on and between the guides, such as to be movable to print aprint medium through the printing screen onto a workpiece which issupported therebelow; and a drive unit for driving the assembly alongthe guides in one of first and second, opposite directions.

In one embodiment the support frame has a rectangular shape in plan viewand includes first and second frame support elements at each of theopposite sides thereof which receive and support the opposite sides ofthe printing screen.

In a further aspect the present invention provides a method of screenprinting a print medium onto a workpiece through a printing screen whichincludes a screen element having a pattern of printing aperturestherein, the method comprising the steps of: providing a print headassembly comprising: first and second carriages which are movablysupported on respective ones of first and second guides; and first andsecond print head units which are mounted in adjacent, parallel relationto and between the carriages, wherein the print head units each comprisefirst and second print head actuators which are mounted on respectiveones of the carriages, and a support member which extends between theprint head actuators, such as to be movable vertically thereby;attaching printing elements, as one of a squeegee blade or a flood bar,to the support members of the print head units; and operating theassembly to print on a workpiece which is supported below the printingscreen.

In one embodiment the guides comprise horizontal guides, preferablylinear bearings.

In one embodiment the print head actuators of each print head unit areoperable to provide for a controlled downward force to the respectivesupport member, thereby enabling application of a controlled downwardforce to the printing element which is attached to the support member.

In one embodiment the controlled downward force is in the range of from0 to about 15 N, preferably with an accuracy of less than about 1%.

In one embodiment the print head actuators comprise low-frictionactuators.

In one embodiment the print head actuators comprise low-frictionpneumatic actuators.

In one embodiment the print head actuators each comprise a cylinder, afloating piston which is movably disposed within the cylinder andincludes an attachment element which is attached to one end of thesupport member, and at least one fluid port to which a fluid pressure isapplied to one of raise or lower the floating piston.

In one embodiment the print head units each further comprise verticalguides, preferably linear bearings, which guide the respective supportmember vertically, such that substantially no horizontal force componentis transmitted to the respective print head actuators.

In one embodiment the support member comprises a substantially U-shapedmember.

In one embodiment the U-shaped member defines a substantially clearopening when viewed laterally.

In one embodiment the support member includes first and second bracketelements which are coupled to respective ones of the respective firstand second print head actuators and extend vertically adjacentrespective inner sides of the print head actuators, and a horizontal barelement which extends between the opposite, lower ends of the bracketelements.

In one embodiment the print head units each comprise a stop adjustmentmechanism, which provides for setting of upper and lower limits tovertical movement of the respective print head actuators.

In one embodiment the stop adjustment mechanism comprises first andsecond stop adjusters, preferably screw adjusters, at each end of thesupport member, and the method further comprises the step of: adjustingthe first stop adjusters to set a lower limit to vertical movement ofthe respective print head actuators and the second stop adjusters to setan upper limit to vertical movement of the respective print headactuators.

In one embodiment the method further comprises the step of: dis-engagingthe second stop adjusters to allow the respective support member to beraised well clear of the printing screen, thereby facilitating removalof the printing screen.

In one embodiment the support members of the print head units aredisposed in spaced relation, and the method further comprises the stepof: introducing print medium from above and between the support members.

In one embodiment the support member of the first print head unit has asqueegee blade attached thereto and the support member of the secondprint head unit has a flood bar attached thereto, and the print headassembly operating step comprises the steps of: controlling the printhead actuators of the first print head unit to bring the squeegee bladeinto operative contact with the screen element and apply a requireddownward force thereto for printing; controlling the print headactuators of the second print head unit to raise the flood bar into aninoperative position; driving the assembly over the surface of thescreen element in a first direction to print deposits of the printmedium onto the workpiece which is supported below the printing screen;controlling the print head actuators of the first print head unit toraise the squeegee blade to an inoperative position; controlling theprint head actuators of the second print head unit to lower the floodbar to an operative position just clear of the screen element; anddriving the assembly over the surface of the screen element in a second,opposite direction to apply a film of the print medium over the surfaceof the screen element.

In another embodiment the support members of the first and second printhead units each have a squeegee blade attached thereto which togetherdefine a cavity, and the print head assembly operating step comprisesthe steps of: controlling the print head actuators of the first printhead unit to bring the squeegee blade attached thereto into operativecontact with the screen element and apply a required downward forcethereto for printing; controlling the print head actuators of the secondprint head unit to raise the squeegee blade attached thereto just clearof the screen element; driving the assembly over the surface of thescreen element in a first direction to print deposits of the printmedium onto the workpiece which is supported below the printing screen;controlling the print head actuators of the first print head unit toraise the squeegee blade attached thereto to a position just clear ofthe screen element; controlling the print head actuators of the secondprint head unit to lower the squeegee blade attached thereto intooperative contact with the screen element and apply a required downwardforce thereto for printing; and driving the assembly over the surface ofthe screen element in a second, opposite direction to print deposits ofthe print medium onto the workpiece which is supported below theprinting screen.

In one embodiment the method further comprises the step of: maintainingthe cavity between the squeegee blades with a volume of print mediumwhich is such that the print medium extends at least across the cavityin contact with each of the squeegee blades.

In another embodiment the method further comprises the step of:maintaining the cavity between the squeegee blades substantially full ofprint medium.

In one embodiment the print medium is a fluid material, preferably aliquid ink.

In another embodiment the print medium is a thick viscous material,preferably a solder paste.

In yet another aspect the present invention provides a print headassembly for printing a print medium onto a workpiece through a printingscreen which includes a screen element having a pattern of printingapertures therein, the assembly comprising a print head unit mountableto and between first and second carriages, wherein the print head unitcomprises first and second print head actuators which are mountable onrespective ones of the carriages, and a support member which extendsbetween the print head actuators, such as to be movable verticallythereby, and in use supports a printing element.

In one embodiment the guides comprise horizontal guides, preferablylinear bearings.

In one embodiment the print head actuators are operable to provide for acontrolled downward force to the support member, thereby enablingapplication of a controlled downward force to a printing element whenattached to the support member.

In one embodiment the controlled downward force is in the range of from0 to about 15 N, preferably with an accuracy of less than about 1%.

In one embodiment the print head actuators comprise low-frictionactuators.

In one embodiment the print head actuators comprise low-frictionpneumatic actuators.

In one embodiment the print head actuators each comprise a cylinder, afloating piston which is movably disposed within the cylinder andincludes an attachment element which is attached to one end of thesupport member, and at least one fluid port to which a fluid pressure isapplied to one of raise or lower the floating piston.

In one embodiment the print head unit further comprises vertical guides,preferably linear bearings, which guide the support member vertically,such that substantially no horizontal force component is transmitted tothe print head actuators.

In one embodiment the support member comprises a substantially U-shapedmember.

In one embodiment the U-shaped support member defines a substantiallyclear opening when viewed laterally.

In one embodiment the support member includes first and second bracketelements which are coupled to respective ones of the print headactuators and extend vertically adjacent respective inner sides of theprint head actuators, and a horizontal bar element which extends betweenthe opposite, lower ends of the bracket elements.

In one embodiment the print head unit comprises a stop adjustmentmechanism, which provides for setting of upper and lower limits tovertical movement of the print head actuators.

In one embodiment the stop adjustment mechanism comprises first andsecond stop adjusters, preferably screw adjusters, at each end of thesupport member, with the first stop adjusters setting a lower limit tovertical movement of the print head actuators and the second stopadjusters setting an upper limit to vertical movement of the print headactuators.

In one embodiment the second stop adjusters can be dis-engaged to allowthe support member to be raised well clear of the printing screen,thereby facilitating removal of the printing screen.

In still another aspect the present invention provides a screen printingsystem, comprising: a support frame which supports a printing screen andincludes first and second guides; the above-described assembly which issupported on and between the guides, such as to be movable to print aprint medium through the printing screen onto a workpiece which issupported therebelow; and a drive unit for driving the assembly alongthe guides in one of first and second, opposite directions.

In one embodiment the support frame has a rectangular shape in plan viewand includes first and second frame support elements at each of theopposite sides thereof which receive and support the opposite sides ofthe printing screen.

In still another aspect the present invention provides a method ofscreen printing a print medium onto a workpiece through a printingscreen which includes a screen element having a pattern of printingapertures therein, the method comprising the steps of: providing a printhead assembly comprising: first and second carriages which are movablysupported on respective ones of first and second guides; and a printhead unit which is mounted to and between the carriages, wherein theprint head unit comprises first and second print head actuators whichare mounted on respective ones of the carriages, and a support memberwhich extends between the print head actuators, such as to be movablevertically thereby; attaching a printing element to the support memberof the print head unit; and operating the assembly to print on aworkpiece which is supported below the printing screen.

In one embodiment the guides comprise horizontal guides, preferablylinear bearings.

In one embodiment the print head actuators are operable to provide for acontrolled downward force to the support member, thereby enablingapplication of a controlled downward force to the printing element whichis attached to the support member.

In one embodiment the controlled downward force is in the range of from0 to about 15 N, preferably with an accuracy of less than about 1%.

In one embodiment the print head actuators comprise low-frictionactuators.

In one embodiment the print head actuators comprise low-frictionpneumatic actuators.

In one embodiment the print head actuators each comprise a cylinder, afloating piston which is movably disposed within the cylinder andincludes an attachment element which is attached to one end of thesupport member, and at least one fluid port to which a fluid pressure isapplied to one of raise or lower the floating piston.

In one embodiment the print head unit further comprises vertical guides,preferably linear bearings, which guide the support member vertically,such that substantially no horizontal force component is transmitted tothe print head actuators.

In one embodiment the support member comprises a substantially U-shapedmember.

In one embodiment the U-shaped member defines a substantially clearopening when viewed laterally.

In one embodiment the support member includes first and second bracketelements which are coupled to respective ones of the respective firstand second print head actuators and extend vertically adjacentrespective inner sides of the print head actuators, and a horizontal barelement which extends between the opposite, lower ends of the bracketelements.

In one embodiment the print head unit comprises a stop adjustmentmechanism, which provides for setting of upper and lower limits tovertical movement of the print head actuators.

In one embodiment the stop adjustment mechanism comprises first andsecond stop adjusters, preferably screw adjusters, at each end of thesupport member, and further comprising the step of: adjusting the firststop adjusters to set a lower limit to vertical movement of the printhead actuators and the second stop adjusters to set an upper limit tovertical movement of the print head actuators.

In one embodiment the method further comprises the step of: dis-engagingthe second stop adjusters to allow the support member to be raised wellclear of the printing screen, thereby facilitating removal of theprinting screen.

In still another aspect the present invention provides a print headassembly for printing a print medium onto a workpiece through a printingscreen which includes a screen element having a pattern of printingapertures therein, the assembly comprising a print head unit mountableto and between first and second carriages, wherein the print head unitcomprises a support member which extends between the carriages and inuse supports a printing element, and the support member comprises asubstantially U-shaped member.

In one embodiment the support member defines a substantially clearopening when viewed laterally.

In one embodiment the guides comprise horizontal guides, preferablylinear bearings.

In one embodiment the print head unit further comprises first and secondprint head actuators which are mountable on respective ones of thecarriages, and the support member extends between the print headactuators, such as to be movable vertically by the print head actuators.

In one embodiment the print head actuators are operable to provide for acontrolled downward force to the support member, thereby enablingapplication of a controlled downward force to a printing element whenattached to the support member.

In one embodiment the controlled downward force is in the range of from0 to about 15 N, preferably with an accuracy of less than about 1%.

In one embodiment the print head actuators comprise low-frictionactuators.

In one embodiment the print head actuators comprise low-frictionpneumatic actuators.

In one embodiment the print head actuators each comprise a cylinder, afloating piston which is movably disposed within the cylinder andincludes an attachment element which is attached to one end of thesupport member, and at least one fluid port to which a fluid pressure isapplied to one of raise or lower the floating piston.

In one embodiment the print head unit further comprises vertical guides,preferably linear bearings, which guide the support member vertically,such that substantially no horizontal force component is transmitted tothe print head actuators.

In one embodiment the support member includes first and second bracketelements which are coupled to respective ones of the print headactuators and extend vertically adjacent respective inner sides of theprint head actuators, and a horizontal bar element which extends betweenthe opposite, lower ends of the bracket elements.

In one embodiment the print head unit further comprises a stopadjustment mechanism, which provides for setting of upper and lowerlimits to vertical movement of the print head actuators.

In one embodiment the stop adjustment mechanism comprises first andsecond stop adjusters, preferably screw adjusters, at each end of thesupport member, with the first stop adjusters setting a lower limit tovertical movement of the print head actuators and the second stopadjusters setting an upper limit to vertical movement of the print headactuators.

In one embodiment the second stop adjusters can be dis-engaged to allowthe support member to be raised well clear of the printing screen,thereby facilitating removal of the printing screen.

In a still further aspect the present invention provides a screenprinting system, comprising: a support frame which supports a printingscreen and includes first and second guides; the above-describedassembly which is supported on and between the guides, such as to bemovable to print a print medium through the printing screen onto aworkpiece which is supported therebelow; and a drive unit for drivingthe assembly along the guides in one of first and second, oppositedirections.

In one embodiment the support frame has a rectangular shape in plan viewand includes first and second frame support elements at each of theopposite sides thereof which receive and support the opposite sides ofthe printing screen.

In a yet still further aspect the present invention provides a screenprinting system, comprising: a support frame which supports a printingscreen and includes first and second guides at opposite sides thereof;and a print head assembly which is supported on and between the guides,such as to be movable to print a print medium through the printingscreen onto a workpiece which is supported therebelow, the assemblycomprising a print head unit mountable to and between first and secondcarriages, wherein the print head unit comprises a support member whichincludes a support element which extends between the carriages and inuse supports a printing element, and, in an operative position, asubstantial part of the support element is located below an uppersurface of the support frame.

In one embodiment the support frame has a rectangular shape in plan viewand includes first and second frame support elements at each of theopposite sides thereof which receive and support the opposite sides ofthe printing screen.

In one embodiment the system further comprises: a drive unit for drivingthe assembly along the guides in one of first and second, oppositedirections.

In one embodiment the support member defines a substantially clearopening when viewed laterally.

In one embodiment the guides comprise horizontal guides, preferablylinear bearings.

In one embodiment the print head unit further comprises first and secondprint head actuators which are mountable on respective ones of thecarriages, and the support member extends between the print headactuators, such as to be movable vertically by the print head actuators.

In one embodiment the print head actuators are operable to provide for acontrolled downward force to the support member, thereby enablingapplication of a controlled downward force to a printing element whenattached to the support member.

In one embodiment the controlled downward force is in the range of from0 to about 15 N, preferably with an accuracy of less than about 1%.

In one embodiment the print head actuators comprise low-frictionactuators.

In one embodiment the print head actuators comprise low-frictionpneumatic actuators.

In one embodiment the print head actuators each comprise a cylinder, afloating piston which is movably disposed within the cylinder andincludes an attachment element which is attached to one end of thesupport member, and at least one fluid port to which a fluid pressure isapplied to one of raise or lower the floating piston.

In one embodiment the print head unit further comprises vertical guides,preferably linear bearings, which guide the support member vertically,such that substantially no horizontal force component is transmitted tothe print head actuators.

In one embodiment the support member includes first and second bracketelements which are coupled to respective ones of the print headactuators and extend vertically adjacent respective inner sides of theprint head actuators, and the support element comprises a horizontal barelement which extends between the opposite, lower ends of the bracketelements.

In one embodiment the print head unit further comprises a stopadjustment mechanism, which provides for setting of upper and lowerlimits to vertical movement of the print head actuators.

In one embodiment the stop adjustment mechanism comprises first andsecond stop adjusters, preferably screw adjusters, at each end of thesupport member, with the first stop adjusters setting a lower limit tovertical movement of the print head actuators and the second stopadjusters setting an upper limit to vertical movement of the print headactuators.

In one embodiment the second stop adjusters can be dis-engaged to allowthe support member to be raised well clear of the printing screen,thereby facilitating removal of the printing screen.

Preferred embodiments of the present invention will now be describedhereinbelow by way of example only with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates a perspective view of a prior art screen printingsystem;

FIGS. 2( a) and (b) illustrate the operation of the print head assemblyof the screen printing system of FIG. 1 in first and second oppositeprint strokes;

FIG. 3 illustrates a perspective view of a screen printing system inaccordance with a preferred embodiment of the present invention;

FIG. 4 illustrates a first vertical sectional view (along section I-I inFIG. 3) of the screen printing system of FIG. 3;

FIG. 5 illustrates a second vertical sectional view (along section II-IIin FIG. 3) of the screen printing system of FIG. 3;

FIGS. 6( a) and (b) illustrate the operation of the print head assemblyof the screen printing system of FIG. 3 in a first, print stroke and asecond, return stroke, respectively; and

FIGS. 7( a) and (b) illustrate the operation of the print head assemblyof an alternative embodiment of the screen printing system of FIG. 3 infirst and second opposite print strokes, respectively.

FIGS. 3 to 6 illustrate a screen printing system in accordance with apreferred embodiment of the present invention, which is configured toreceive a printing screen 105 and print on workpieces through theprinting screen 105.

In this embodiment the printing screen 105 comprises a frame 107, whichhas a rectangular shape in plan view, and a screen element 109, here inthe form of a solid sheet, either formed of a metal or plasticsmaterial, or a coated mesh, which is mounted under tension to the frame107 and includes a pattern of printing apertures 110, which defines aprint area, through which a print medium is printed onto a workpiecesupported therebelow.

The screen printing system comprises a rigid support frame 111 whichsupports the printing screen 105, first and second horizontal guides117, 117 which are disposed in parallel relation on the support frame111 at opposite sides of the printing screen 105, a print head assembly119 which is supported on the horizontal guides 117, 117 such as to bemovable in a horizontal plane and print a print medium through theprinting screen 105 onto a workpiece which is supported therebelow, anda drive unit 121 for driving the print head assembly 119 along thehorizontal guides 117, 117 in one of first and second, oppositedirections D1, D2.

In this embodiment the support frame 111 has a rectangular shape in planview and includes first and second frame support elements 125, 125, hereledges, at the opposite sides thereof, here at the lower edges, whichreceive and support the opposite sides of the frame 107 of the printingscreen 105.

In this embodiment, as illustrated in FIG. 5, the opposite sides of thesupport frame 111 include inwardly-facing locating elements 131, whichact to locate the printing screen 105 within the support frame 111. Inthis embodiment the locating elements 131 on one side of the supportframe 111 are resiliently biased, here by spring elements 133, such asto bias the frame of the printing screen 105 against the locatingelements 131 on the other side of the support frame 111.

In this embodiment the screen printing system further comprises a screenlocking mechanism 135 for locking the printing screen 105 in a fixedposition to the support frame 111.

In this embodiment, as illustrated in FIG. 5, the screen lockingmechanism 135 comprises a plurality of clamp members 137 which aredisposed in opposed relation to the frame support elements 125, 125 andoperable to clamp the frame 107 of the printing screen 105 to the framesupport elements 125, 125, and at least one, in this embodiment aplurality of actuators 139, here pneumatic actuators, for moving theclamp members 137 to clamp the printing screen 105 to the support frame111.

In this embodiment the horizontal guides 117, 117 on the support frame111 comprise linear bearings to which opposite ends of the print headassembly 119 are movably supported.

In this embodiment the print head assembly 119 comprises first andsecond carriages 141, 141, which are mounted on respective ones of thefirst and second horizontal guides 117, 117, and first and second printhead units 143, 145, which are mounted in adjacent, parallel relation toand between the first and second carriages 141, 141.

In this embodiment the first and second carriages 141, 141 each includea coupling element 142, here including a toothed section, for engagingrespective drive elements 185, 185 of the drive unit 121, as will bedescribed in more detail hereinbelow.

The print head units 143, 145 each comprise first and second print headactuators 147, 147 which are mounted on respective ones of the first andsecond carriages 141, 141, a support member 151 which extends betweenthe print head actuators 147, 147, such as to be movable verticallythereby, and supports a printing element, either as a squeegee blade 181or a flood blade/bar 183, as will be described in more detailhereinbelow, and first and second vertical guides 152, 152, which guidethe support member 151 in a vertical orient.

By supporting the print head actuators 147, 147 directly on the supportframe 111, and not on a necessarily-bulky gantry as in the prior art,the support members 151, 151 of the first and second print head units143, 145 can be of much reduced dimension, which reduces weight and alsoenables the support members 151, 151 to be located close to the printingscreen 105 and not extend over a significant height, here having anupper height which is lower than the upper surface of the support frame111, which improves line-of-sight visibility. In this embodiment thesupport members 151, 151 present a substantially open section betweenthe support frame 111 when viewed horizontally.

In this embodiment the print head actuators 147, 147 compriselow-friction pneumatic actuators (Type MQQ TB 20-10D, as manufactured bySMC Pneumatics (UK), Milton Keynes, United Kingdom), which comprise acylinder 153, a floating piston 154 which is movably disposed within thecylinder 153 and includes a rod guide 155 which extends vertically fromthe cylinder 153 and is moved by movement of the floating piston 154,and first and second fluid ports 157, 159 to which a fluid pressure,here a pneumatic pressure, as one of a positive pressure or a negativepressure (vacuum), is applied to one of raise or lower the floatingpiston 154 and the vertically-extending rod guide 155.

In this embodiment the respective ones of the first and second fluidports 157, 159 of the print head actuators 147, 147 of the respectiveones of the first and second print head units 143, 145 are commonlycoupled, such that the print head actuators 147, 147 of the respectiveone of the first and second print head units 143, 145 are operated inunison.

In this embodiment the fluid supply is a pneumatic, compressed airsupply which is switchably delivered to the first and second fluid ports157, 159 of the print head actuators 147, 147 of the respective ones ofthe first and second print head units 143, 145 through a precisionpressure regulator (Type ITV1050 as manufactured by SMC Pneumatics (UK),Milton Keynes, United Kingdom).

In this embodiment the first and second vertical guides 152, 152comprise linear bearings to which opposite ends of the support member151 are movably supported, such as to ensure that no horizontal forcecomponent is transmitted to the print head actuators 147, 147, whichwould otherwise act to provide a frictional resistance to the print headactuators 147, 147, which would impair the required precision pressurecontrol from the print head actuators 147, 147.

In this embodiment the support member 151 comprises a substantiallyU-shaped member, which includes first and second bracket elements 161,161 which are coupled to respective ones of the print head actuators147, 147 and extend vertically adjacent the respective inner sides ofthe print head actuators 147, 147, and a horizontal bar element 163which extends between the opposite, lower ends of the bracket elements161, 161.

With this configuration, the U-shaped profile of the support members151, 151 of the first and second print units 143, 145 provides anoperator with much improved visibility of the surface of the printingscreen 105, which is particularly advantageous in monitoring the stateof the printing screen 105 and the print medium.

In this embodiment the first and second print head units 143, 145 eachcomprise a stop adjustment mechanism 171, which provides for setting ofupper and lower limits to movement of the pistons 154, 154 of the printhead actuators 147, 147.

In this embodiment the stop adjustment mechanism 171 comprises first andsecond screw adjusters 173, 175 at each end of the support member 151,with the first screw adjusters 173, 173 setting a lower limit tomovement of the pistons 154, 154 of the print head actuators 147, 147and the second screw adjusters 175, 175 setting an upper limit tomovement of the pistons 154, 154 of the print head actuators 147, 147.

In one embodiment the print head actuators 147, 147 can be provided aslonger stroke actuators, which, by dis-engaging the upper limit screwadjusters 175, 175, would allow the respective support member 151 andthe printing element attached thereto to be raised well clear of theprinting screen 105, thereby facilitating removal of the printing screen105.

In this embodiment the support member 151 of the first print head unit143 has a squeegee blade 181 attached thereto and the support member 151of the second print head unit 145 has a flood bar 183 attached thereto.

With no pressure applied to the print head actuators 147, 147 of thefirst print head unit 143, the squeegee blade 181 will rest on thesurface of the screen element 109 of the printing screen 105 and apply aforce to the screen element 109 which is determined by the combinedweight of the squeegee blade 181, the support member 151 and thefloating piston 154 of each of the print head actuators 147, 147.

By applying a positive fluid pressure to the first fluid ports 157, 157or a negative fluid pressure to the second fluid ports 159, 159 of theprint head actuators 147, 147 of the first print head unit 143, theforce applied by the squeegee blade 181 to the screen element 109 can bereduced, and, through the application of sufficient fluid pressure, thesqueegee blade 181 can be lifted clear of the screen element 109, up tothe upper limit set by the second screw adjusters 175, 175 of the stopadjustment mechanism 171.

Alternatively, by applying a negative fluid pressure to the first fluidports 157, 157 or a positive fluid pressure to the second fluid ports159, 159 of the print head actuators 147, 147 of the first print headunit 143, the force applied by the squeegee blade 181 to the screenelement 109 can be increased.

In this way, by controlled application of fluid pressure to respectiveones of the first fluid ports 157, 157 or the second fluid ports 159,159 of the print head actuators 147, 147 of the first print head unit143, the squeegee blade 181 can apply any required downward force F tothe screen element 109, in this embodiment in the range of from 0 toabout 15 N, with an accuracy of about 1% as illustrated in FIG. 6( a),or the squeegee blade 181 can be held clear of the screen element 109 inan inoperative position, as illustrated in FIG. 6( b).

In an alternative embodiment sufficient fluid pressure could be appliedto the first fluid ports 157, 157 of the first print head unit 143 as tocounteract the dead weight of the squeegee mechanism, and a controlledfluid pressure could be applied to the second fluid ports 159, 159 ofthe first print head unit 143 as to apply a required downward force tothe squeegee blade 181.

With no pressure applied to the print head actuators 147, 147 of thesecond print head unit 145, the pistons 154, 154 of the print headactuators 147, 147 will be at the lower stop positions as defined by thefirst screw adjusters 173, 173 of the stop adjustment mechanism 171,such that the flood bar 183 is held just clear of the screen element 109in a re-coating position, as illustrated in FIG. 6( b).

By applying a positive fluid pressure to the second fluid ports 159, 159or a negative fluid pressure to the first fluid ports 157, 157 of theprint head actuators 147, 147 of the second print head unit 145, theflood bar 183 is locked in the re-coating position. When using viscousliquid inks, the printing screen 105 commonly uses a coated mesh for thescreen element 109 and it is desirable to maintain the image area of thescreen element 109, as defined by the pattern of printing apertures 110,covered with a thin film of ink in order to prevent the ink from dryingin the screen element 109 and blocking the pattern of printing apertures110. By positioning the flood bar 183 in the re-coating position whenreturning the print head assembly 119 to the initial, starting positionin each return stroke, the flood bar 183 acts to re-coat the surface ofthe screen element 109 with a thin film of the print medium, asillustrated in FIG. 6( b).

By applying a positive fluid pressure to the first fluid ports 157, 157or a negative fluid pressure to the second fluid ports 159, 159 of theprint head actuators 147, 147 of the second print head unit 145, theflood bar 183 can be raised further clear of the screen element 109, asdefined by the upper limit set by the second screw adjusters 175, 175 ofthe stop adjustment mechanism 171, as illustrated in FIG. 6( a).

In this embodiment the drive unit 121 comprises first and second driveelements 185, 185, here toothed belts, which are attached to thecoupling elements 142, 142 of respective ones of the first and secondcarriages 141, 141, and a drive motor 187 which is operable to drive thedrive elements 185, 185 and hence the first and second carriages 141,141. In this embodiment the drive elements 185, 185 are commonly coupledby a drive shaft 189, and the drive motor 187 is coupled to one driveelement 185, such that operation of the drive motor 187 commonly drivesthe drive elements 185, 185 through the drive shaft 189.

Operation of the print head assembly 119 is illustrated in FIGS. 6( a)and (b).

In a print stroke, as illustrated in FIG. 6( a), in which the print headassembly 119 is moved in a first direction D1, the squeegee blade 181 ofthe first print head unit 143 is in contact with the screen element 109and loaded with a required downward force F by the print head actuators147, 147 of the first print head unit 143, such as to push the printmedium PM forwardly of the face of the squeegee blade 181 and throughthe pattern of printing apertures 110 in the screen element 109 asdeposits onto a workpiece W which is supported therebelow, and the floodbar 183 of the second print head unit 145 is held in the raised,inoperative position by the print head actuators 147, 147 of the secondprint head unit 145.

In a return stroke, as illustrated in FIG. 6( b), in which the printhead assembly 119 is moved in a second direction D2, the squeegee blade181 of the first print head unit 143 is raised out of contact with thescreen element 109 and held in a raised, inoperative position byoperation of the print head actuators 147, 147 of the first print headunit 143, and the flood bar 183 of the second print head unit 145 islowered and held in the operative, re-coating position, such as tore-coat the surface of the screen element 109 with a thin film of theprint medium PM.

In an alternative embodiment, as illustrated in FIGS. 7( a) and (b),which, for example, is for the printing of thick viscous materials, thesupport member 151 of the second print head unit 145 is fitted with asqueegee blade 181 in place of a flood bar 183, and the second printhead unit 145 operated in the same manner as the first print head unit143, as described hereinabove.

In this embodiment the upper ends of the squeegee blades 181, 181 arespaced apart sufficiently as to allow for the introduction of printmedium PM into the cavity which is defined therebetween. In this way,the cavity between the squeegee blades 181, 181 can be maintained with ahigh fill volume of the print medium PM, and at least such that theprint medium PM extends across the cavity in contact with each of thesqueegee blades 181, 181. With this configuration, circulation of theprint medium PM is achieved, which provides for a sufficient level ofshear thinning as to enable printing at high print speeds, and higherprint speeds than existing print heads.

In this embodiment the print head assembly 119 includes first and secondend checks 191, 191, which are coupled to respective ends of thesqueegee blades 181, 181, such as to retain the print medium PM betweenthe squeegee blades 181, 181.

In this embodiment the end checks 191, 191 are fixed with fixingelements to the ends of the squeegee blades 181, 181, and each includeslotted apertures, which extend in a vertical orient and allow forrelative vertical movement of the squeegee blades 181, 181.

Operation of this alternative print head assembly 119 is illustrated inFIGS. 7( a) and (b). In this embodiment the print head assembly 119 isreciprocatingly movable in first and second print strokes in oppositedirections D1, D2.

In a first, out print stroke, as illustrated in FIG. 7( a), in which theprint head assembly 119 is moved in a first direction D1, the rearsqueegee blade 181 of the first print head unit 143 is in contact withthe screen element 109 and loaded with a required downward force F bythe print head actuators 147, 147 of the first print head unit 143, suchas to push the print medium PM forwardly of the face of the squeegeeblade 181 and through the pattern of printing apertures 110 in thescreen element 109 as deposits onto a workpiece W which is supportedtherebelow, and the forward squeegee blade 181 of the second print headunit 145 is held in a raised position just clear of the screen element109 by the print head actuators 147, 147 of the second print head unit145. The forward squeegee blade 181 of the second print head unit 145 isin close proximity to the screen element 109, but sufficiently spaced asto prevent contact with the screen element 109 during the out printstroke. In one embodiment the lower edge of the forward squeegee blade181 is spaced by no more than about 1 mm from the screen element 109,and preferably by no more than about 0.5 mm. This configuration is inmarked contrast to the prior art squeegee systems, in which the forwardsqueegee is maintained well clear of the rear, operative squeegee.

In a second, return print stroke, as illustrated in FIG. 7( b), in whichthe print head assembly 119 is moved in a second, opposite direction D2,the rear squeegee blade 181 of the second print head unit 145 is incontact with the screen element 109 and loaded with a required downwardforce F by the print head actuators 147, 147 of the second print headunit 145, such as to push the print medium PM forwardly of the face ofthe squeegee blade 181 and through the pattern of printing apertures 110in the screen element 109 as deposits onto a workpiece W which issupported therebelow, and the forward squeegee blade 181 of the firstprint head unit 143 is held in a raised position just clear of thescreen element 109 by the print head actuators 147, 147 of the firstprint head unit 143. Similarly to the configuration for the first, outprint stroke, the forward squeegee blade 181 of the first print headunit 143 is in close proximity to the screen element 109, butsufficiently spaced as to prevent contact with the screen element 109during the return print stroke. In one embodiment the lower edge of theforward squeegee blade 181 is spaced by no more than about 1 mm from thescreen element 109, and preferably by no more than about 0.5 mm.

The present inventor has established surprisingly that thisconfiguration of the squeegee blades 181, 181, in which the forwardsqueegee blade 181 in the direction of movement is raised just clear ofthe screen element 109, in combination with the provision of a volume ofprint medium PM in the cavity between the squeegee blades 181, 181 whichsubstantially fills the cavity, provides for a performance at leastequivalent to the existing enclosed screen printing heads.

Finally, it will be understood that the present invention has beendescribed in its preferred embodiments and can be modified in manydifferent ways without departing from the scope of the invention asdefined by the appended claims.

1. A print head assembly for printing a print medium onto a workpiecethrough a printing screen which includes a screen element having apattern of printing apertures therein, the assembly comprising: firstand second carriages which are in use movably supported on respectiveones of first and second guides; and first and second print head unitswhich are mounted in adjacent, parallel relation to and between thecarriages, wherein the print head units each comprise first and secondprint head actuators which are mounted on respective ones of thecarriages, and a support member which extends between the print headactuators, such as to be movable vertically thereby, and in use supportsa printing element.
 2. The assembly of claim 1, wherein the guidescomprise horizontal guides, preferably linear bearings.
 3. The assemblyof claim 1, wherein the print head actuators of each print head unit areoperable to provide for a controlled downward force to the respectivesupport member, thereby enabling application of a controlled downwardforce to a printing element when attached to the support member.
 4. Theassembly of claim 3, wherein the controlled downward force is in therange of from 0 to about 15 N, preferably with an accuracy of less thanabout 1%.
 5. The assembly of claim 3, wherein the print head actuatorscomprise low-friction actuators.
 6. The assembly of claim 5, wherein theprint head actuators comprise low-friction pneumatic actuators.
 7. Theassembly of claim 3, wherein the print head actuators each comprise acylinder, a floating piston which is movably disposed within thecylinder and includes an attachment element which is attached to one endof the support member, and at least one fluid port to which a fluidpressure is applied to one of raise or lower the floating piston.
 8. Theassembly of claim 1, wherein the print head units each further comprisevertical guides, preferably linear bearings, which guide the respectivesupport member vertically, such that substantially no horizontal forcecomponent is transmitted to the respective print head actuators.
 9. Theassembly of claim 1, wherein the support member comprises asubstantially U-shaped member.
 10. The assembly of claim 9, wherein theU-shaped support member defines a substantially clear opening whenviewed laterally.
 11. The assembly of claim 9, wherein the supportmember includes first and second bracket elements which are coupled torespective ones of the respective first and second print head actuatorsand extend vertically adjacent respective inner sides of the print headactuators, and a horizontal bar element which extends between theopposite, lower ends of the bracket elements.
 12. The assembly of claim1, wherein the print head units each comprise a stop adjustmentmechanism, which provides for setting of upper and lower limits tovertical movement of the respective print head actuators.
 13. Theassembly of claim 12, wherein the stop adjustment mechanism comprisesfirst and second stop adjusters, preferably screw adjusters, at each endof the support member, with the first stop adjusters setting a lowerlimit to vertical movement of the respective print head actuators andthe second stop adjusters setting an upper limit to vertical movement ofthe respective print head actuators.
 14. The assembly of claim 13,wherein the second stop adjusters can be dis-engaged to allow therespective support member to be raised well clear of the printingscreen, thereby facilitating removal of the printing screen.
 15. Theassembly of claim 1, wherein the support members of the print head unitsare disposed in spaced relation, such as to allow for introduction ofprint medium from above therebetween.
 16. The assembly of claim 1,wherein the support member of the first print head unit has a squeegeeblade attached thereto and the support member of the second print headunit has a flood bar attached thereto, and the assembly is movable inoppositely-directed print and return strokes, such that, in the printstroke, the print head actuators of the first print head unit arecontrolled to bring the squeegee blade into operative contact with thescreen element and apply a required downward force thereto for printingand the print head actuators of the second print head unit arecontrolled to raise the flood bar into an inoperative position, and, inthe return stroke, the print head actuators of the first print head unitare controlled to raise the squeegee blade to an inoperative positionand the print head actuators of the second print head unit arecontrolled to lower the flood bar to an operative position just clear ofthe screen element, such as to provide for application of a film of theprint medium over the surface of the screen element.
 17. The assembly ofclaim 1, wherein the support members of the print head units each have asqueegee blade attached thereto, and the assembly is movable in firstand second oppositely-directed print strokes, such that, in the firstprint stroke, the print head actuators of the first print head unit arecontrolled to bring the squeegee blade attached thereto into operativecontact with the screen element and apply a required downward forcethereto for printing and the print head actuators of the second printhead unit are controlled to raise the squeegee blade attached theretojust clear of the screen element, and, in the second print stroke, theprint head actuators of the first print head unit are controlled toraise the squeegee blade attached thereto to a position just clear ofthe screen element and the print head actuators of the second print headunit are controlled to lower the squeegee blade attached thereto intooperative contact with the screen element and apply a required downwardforce thereto for printing.
 18. A screen printing system, comprising: asupport frame which supports a printing screen and includes first andsecond guides; the assembly of claim 1 which is supported on and betweenthe guides, such as to be movable to print a print medium through theprinting screen onto a workpiece which is supported therebelow; and adrive unit for driving the assembly along the guides in one of first andsecond, opposite directions.
 19. (canceled)
 20. A method of screenprinting a print medium onto a workpiece through a printing screen whichincludes a screen element having a pattern of printing aperturestherein, the method comprising the steps of: providing a print headassembly comprising: first and second carriages which are movablysupported on respective ones of first and second guides; and first andsecond print head units which are mounted in adjacent, parallel relationto and between the carriages, wherein the print head units each comprisefirst and second print head actuators which are mounted on respectiveones of the carriages, and a support member which extends between theprint head actuators, such as to be movable vertically thereby;attaching printing elements, as one of a squeegee blade or a flood bar,to the support members of the print head units; and operating theassembly to print on a workpiece which is supported below the printingscreen. 21-40. (canceled)
 41. A print head assembly for printing a printmedium onto a workpiece through a printing screen which includes ascreen element having a pattern of printing apertures therein, theassembly comprising a print head unit mountable to and between first andsecond carriages, wherein the print head unit comprises first and secondprint head actuators which are mountable on respective ones of thecarriages, and a support member which extends between the print headactuators, such as to be movable vertically thereby, and in use supportsa printing element. 42.-102. (canceled)